The present invention relates to a film thickness measuring apparatus for measuring the thickness of a thin film formed on a substrate, such as a semiconductor wafer, and also to a substrate processing method and substrate processing apparatus that are provided with a film thickness measuring function.
In general, when a semiconductor device, such as an integrated circuit, is manufactured, desired circuit elements are formed by super fine working technology, with a film forming process, an etching process and other processes being repeated. With the recent development in technology, there is an increased demand for larger storage capacity and higher processing speed of microprocessors in the field of semiconductor devices. It is therefore necessary to provide developing devices having higher integration density and super fine structure.
Under these circumstances, the thickness control of films of various kinds has to be performed with high accuracy. By way of example, reference will be made to the gate oxide film of a transistor used in a microprocessor. When a thermal oxide film, with which to form the gate oxide film, is, for example, 20 nm, there is a thickness allowance of .+-. about 0.5 nm in the conventional art. In other words, the thickness variation within this range does not result in a change in the characteristics. In the next-generation devices, however, thin films which are 3 nm or less are required. In the case of such very thin films, the allowable thickness variation is less than .+-.0.05 nm, and highly accurate thickness control is therefore necessary.
One of the problems that has to be solved in connection with the thin films is related to measurement technology. Let us consider the case where a thin film is formed on a semiconductor wafer to have a predetermined thickness and is then measured by the same film thickness measuring apparatus. In this case, the measurement will gradually increase with time, and it is impossible to know the actual thickness of the thin film. For example, when a thermal oxide film having a thickness of 10 nm was formed on a semiconductor wafer and its thickness was measured several times after it was left to stand in a clean room, the results shown in FIG. 17 were obtained. FIG. 17 is a characteristic graph wherein the time (minutes) for which the film is left to stand after film formation is plotted against an abscissa, and the measured thickness (nm) is plotted against an ordinate. That is, FIG. 17 shows how the time is related to the measurement of the film thickness. In FIG. 17, the plotted white squares indicate data obtained by film thickness measuring apparatuses (FEIII) based on elliptical polarization method. In other words, they indicate how the measurement of the thickness of the thin film formed on the surface of the semiconductor wafer is dependent on the time. As is apparent from FIG. 17, the value measured after a lapse of 3 minutes is about 10 nm, which is a reliable value. However, the measurement value increases to about 10.1 nm when 30 minutes have passed from the film formation, and to about 10.3 nm when 300 minutes have passed.
As described, the next-generation devices, the allowable film thickness measurement variation range of which is .+-.0.05 nm or so, cannot be measured reliably.
As one of the factors that vary the film thickness measurement, organic matter attaching to the film surface is thought of. That is, the organic matter existing in the atmosphere of the film thickness measurement space attaches to the film surface with time. To prevent this attachment of organic matter, the conventional art, such as that disclosed in Jpn. Pat. Appln. KOKAI Publication No. 10-82631, proposes a film thickness measurement space which is provided inside an area separate from the outside air and which is filled with an inert gas, nitrogen gas.
According to this technology, the measurement space must be evacuated of the atmospheric gas and then filled with an inert gas instead. This being so, a large amount of gas is consumed in vain each time a semiconductor wafer is loaded or unloaded, which is uneconomic. In addition, safety measures must be taken, depending upon the kind of gas used.
Technology that employs a vacuum pump to reduce the pressure in a measurement space is known as technology that does not use an inert gas. However, this technology is disadvantageous in that the various driving mechanisms arranged in the measurement space must use a special kind of lubricant, such as vacuum grease, and the use of such lubricant inevitably increases the equipment cost.
Accordingly, the first object of the present invention is to provide a film thickness measuring apparatus which can measure the thickness of a film on a substrate with high accuracy, with no need to introduce an inert gas into a measurement space or to execute evacuation.
Another object of the present invention is to provide a substrate processing apparatus which can measure the thickness of a film on a substrate with high accuracy, ensures substantially stable measurement at all times, and can execute the next processing.